'""I AREA- 1 (N-i.) 



° I I I I I I I I I I n I I I Fi I 



20 <0 60 80 100 120 140 160 



20 -1 



_ AREA - 2 (N=4i) 



T — I — I — r 



I I I 



100 120 140 160 



ALL AREAS 

 COMBINED 



(N-611) 



I I I 



20 40 



I I n 



so 100 



r 



FORK LENGTH (cm) FORK LENGTH (cm) 



Figure 3. — Length-frequency distribution of yellowfin tuna from which stomachs were collected. 



METHODS USED IN THE EVALUATION OF DATA 



The data were evaluated by percentage fi'e- 

 quency of occurrence and percentage of total 

 volume (Reintjes and King, 1953). Statistical 

 analyses were based on Spearman's rank cor- 

 relation test and a paired t-test of difference 

 between means. 



COMPOSITION OF FORAGE ORGANISMS 

 FOUND IN THE STOMACHS OF SKIP- 

 JACK AND YELLOWFIN TUNAS 



The identified food of skipjack and yellowfin 

 tunas consisted chiefly of three major cate- 

 gories — fish, crustaceans, and cephalopods (fig. 

 4). Appendix tables 1 to 6 list all individual 

 food items identified from stomachs of skipjack 

 and yellowfin tunas. These tables include for 

 each area the food item, the number of stomachs 

 in which it occurred, the percentage frequency 

 of occurrence, the displaced volume, and the 



percentage of the total volume examined. The 

 food items are listed in order of frequency of 

 occurrence by families (fishes) or broader cate- 

 gories (mollusks and crustaceans). 



The numbers of taxa found in the stomachs 

 of the two species were similar — 159 in skip- 

 jack tuna and 174 in yellowfin tuna; 102 were 

 in stomachs of both species (59 fishes, 29 

 crustaceans, and 14 mollusks). 



The taxonomic composition of forage orga- 

 nisms differed in the three areas. In areas 1 

 and 2, fish was the most important food item in 

 volume and frequency of occurrence for both 

 species (fig. 4). In area 3, crustaceans led fish 

 and mollusks in frequency of occurrence for 

 both species ; by volume, crustaceans were more 

 important than fish only in the diet of skipjack 

 tuna. Cephalopods (chiefly squid) made up the 

 major portion of mollusks in all areas; by 

 volume, cephalopods were more important than 



448 



U.S. FISH AND WILDLIFE SERVICE 



